1. Field of the Invention
This invention relates to optical storage and, in particular, to optical storage achieved employing holography.
2. Art Background
A variety of approaches have been suggested for the storage of information. Of these approaches, optical storage has long been investigated. One optical approach, holography, although potentially offering high density storage, i.e., greater than 100 bits/.mu.m.sup.2, has never satisfied expectations. In the holographic storage approach, light carrying the information, such as a digital video image, digital data, text, or audio, is caused to intersect a reference light beam in the volume of a recording medium, such as lithium niobate. The resulting interference pattern produced by the interaction of the reference and information light is denominated a hologram. In common practices, holograms are recorded in the medium as a corresponding pattern of changes in a property of the medium, e.g. refractive index or absorption. The hologram is reconstructed by interrogating the medium with the reference light and observing the light, e.g. diffracted light, after its interaction with the medium.
The number of such holograms per unit volume that can be stored in, and ultimately reconstructed from, the medium is a measure of information storage density. The selectivity, i.e. the change in angle, wavelength, position, or other physical parameter required before a new hologram can be recorded and read independently, of the writing/reconstruction technique determines the useable storage density. A typical approach for reconstructing one recorded hologram without interference from another, i.e., for providing selectivity, is denominated Bragg selectivity. In one particular variant of such approach, each hologram is written using a different angle between the information and reference light and then reconstructed at the corresponding reference angle. Generally, the selectivity of such Bragg techniques requires a change of at least .DELTA..theta.=.lambda./L degrees between adjacent holograms to allow independent reconstruction (where .lambda. is the wavelength of the reference light and L is the thickness of the region of intersection between the reference and signal light.) Other methods for providing selectivity are peristrophic and fractal multiplexing. See Curtis et al., Optics Letters, 19 (13), 993 (1994), Psaltis et al., SPIE Proceedings, 963 70: 1988 ICO Topical Meeting on Optical Computing Toulon. respectively, for a description of these techniques. Each technique depends on a change of orientation between the reference beam, signal beam, and/or the recording medium.
Although selectivity is one primary factor affecting signal to noise ratio of the reconstructed hologram, and the storage density of the memory, it is not the only factor. A second primary factor is the total refractive index (or other medium property being relied upon) change producible in the medium. The total refractive index change is, in turn, dependent on the volume of the medium and the absolute value of the refractive index change induced in the composition. Since, for a given composition the maximum possible refractive index change is fixed, increased storage capacity requires an increased volume of the recording medium. Similarly, for a given recording technique selectivity, and thus, storage per unit volume (i.e., density) is again fixed, and thus, increased storage requires increased volume.
To increase density and selectivity for the Bragg approach, holograms have been written in relatively thick materials, i.e., materials having a thickness greater than approximately 1 mm. Nevertheless, a monolithic, thick medium with suitable properties has generally proven difficult to achieve in practice. Such medium should be flat, have a maximum refractive index perturbation, .DELTA.n, greater than 0.0001, have an absorption coefficient less than 2 at the writing wavelength with a thickness deviation typically no greater than approximately 100 .mu.m, have less than 2% shrinkage on exposure, have relatively small thermal coefficient of expansion (less than 500 ppm), and have a sensitivity of greater than 10.sup.-3 per joule. Satisfaction of all these criteria with a single, thick, monolithic structure is extremely difficult to achieve. Attempts have been made to write holograms in stratified structures to effect a thick medium by using a multiplicity of thin layers. Such structures typically include a transparent region, e.g., a glass plate, where no recording occurs (an inactive region) alternating with regions responsive to the recording light (active regions). Such stratified structures have been shown useful for many applications involving the writing and reconstruction of a single holographic optical element. (See, for example, Nordin and Tanguay, Optical Letters, 17, 1709 (1992) and Nordin, et al., Journal of the Optical Society of America A, 9, 2206 (1992).)
The formation and reconstruction of a multitude of holographic structures in a stratified medium, however, has been met with significant obstacles. In such attempts, a Bragg selectivity approach has been employed. The attempted use of Bragg holography in a stratified medium, however, results in unacceptable signal to noise upon reconstruction of holograms that have been stored as close as the total active thickness would, in theory, allow in a correspondingly thick medium that is not stratified. That is, crosstalk noise (noise from other multiplexed holograms) has limited the usefulness of stratified samples with Bragg approaches. The resulting lack of selectivity improvement severely limits the density of stored information. This phenomenon is demonstrated in Stankus, et al., Optics Letters, 19, 1480 (1994). As shown in FIG. 3 of that article, for each hologram, numerous angles will yield some reconstructed output causing undesirable interference. As noted in the Stankus article, the only apparent approach for avoiding such problems is to employ a stratified medium where the active regions are at least 15 times thicker than the inactive regions. Clearly, such approach requires a structure which is either mechanically unstable or one that, because of the thickness of its active regions, yields the same difficulties in producing flatness, uniformity, and the other characteristics required of the active medium.
Thus, a medium and a method of recordation and reconstruction in such medium that yields resolvable high information storage, has been an elusive goal.